The formation of free radical intermediates in the reactions of gaseous NO2 with solid NaCl and NaBr—Atmospheric and toxicological implications

The reaction of gaseous NO2 with alkali halides is of significant interest for its potential to occur in polluted marine atmospheres ((Finlayson-Pitts, 1993), Res. Chem. Int. 19, 235–249). We report here a new paramagnetic signal in NaBr and NaCl formed upon reaction with gaseous NO2 and assigned to a radical anion intermediate of the type [X… NO2]− where X = Br, Cl. This is a new type of Vk center (Castner and Känzig, 1957, J. Phys. Chem. Solids 3, 178–195; Fowler, 1968; Seidel, 1969, in Magnetic Resonance and Radiofrequency Spectroscopy (edited by Averbuch P.), North Holland, Amsterdam, pp. 141–156), which is remarkably stable. Indeed, with NaCl it can be observed for several days following the reaction after handling the sample in room light and ambient air. This intermediate may be responsible at least in part for synergistic effects observed in the deep lung in rats exposed simultaneously to gaseous NO2 and NaCl aerosol (Last and Warren, 1987, Toxicol. Appl. Pharmacol. 90, 34–42). While the gaseous product ClNO was suggested earlier as the causative agent (Last and Warren, 1987, Toxicol. Appl. Pharmacol. 90, 34–42), it is highly reactive with liquid water and will be removed largely in the upper respiratory tract, while small particles are able to penetrate into the deep lung. Given the ubiquitous occurrence (Finlayson-Pitts and Pitts, 1986, Atmospheric Chemistry: Fundamentals and Experimental Techniques, Wiley, New York), of NO2 and airborne salt particles, which occur not only in marine areas but also significant distances inland (Shaw, 1991, J. geophys. Res. 96, 22,369–22,372) as well as in the plumes from oil well burning (Lowenthal et al., 1993, Geophys. Res. Lett. 20, 691–693; Stevens et al., 1993, Wat. Sci. Technol. 27, 223–233; Sheridan et al., 1992, Geophys. Res. Lett. 19, 389–392; Parungo et al., 1992, J. geophys. Res. 97, 15,867–15,882; Cahill et al., 1992, J. geophys. Res. 97, 14,513–14,520) this radical anion could provide a vehicle for delivery of reactive species into the deep lung. These observations are discussed in the context of the mechanism of action of PM10 which has been associated recently with increased mortality (Dockery et al., 1993, New Eng. J. Med. 329, 1753–1759).